1. A sensitive and accurate radioimmunoassay system for the A-I apolipoprotein has been established and applied in studies of normals, subjects with Tangier disease and patients with a variety of hyperlipoproteinemias. ApoLP-A-I concentrations in Tangier patients were shown to be less than 1% of normal and reductions of 50% or greater were demonstrated in Tangier heterozygotes and patients with type V hyperlipoproteinemia. 2. Technical problems in the purification of apoLP-C-II have been resolved and sufficient quantities of the protein have been isolated to permit determination of its amino acid sequence and investigations of its structure-function relationships. 3. Characterization of the plasma lipoproteins and apolipoproteins of Tangier disease is almost complete. The data indicate that high density lipoproteins modulate the catabolism of very low density to low density lipoproteins. 4. Studies on the fractionation and characterization of the hepatic and extrahepatic triglyceride hydrolase enzymes have been initiated with the goal of purifying the enzymes and characterizing their substrate and cofactor specificities. 5. A preliminary survey of the plasma lipoproteins and apolipoproteins in type III hyperlipoproteinemia has been completed. Considerable variability in lipoprotein lipid and protein composition has been found among patients with presumably the same genetic defect.